Externalities: a) Private, external and social costs and benefits

The idea

Understanding externalities requires distinguishing private, external, and social costs and benefits. Private costs are borne directly by the producer or consumer, such as raw material expenses. External costs are negative side-effects imposed on third parties, like pollution affecting health. Social costs represent the total cost to society, calculated as private plus external costs. Private benefits are direct gains, such as personal utility from education. External benefits are positive spillovers to third parties, like a more productive workforce. Social benefits equal private plus external benefits. Market failure occurs when the price mechanism only considers private values, ignoring external ones, leading to resource misallocation.

What earns the marks

• Define private costs and benefits as those directly affecting the economic agents involved in a transaction.
• Define external costs and benefits as the spillover effects on third parties not involved in the transaction.
• Calculate social costs as the sum of private and external costs, and social benefits as the sum of private and external benefits.
• Explain how the divergence between private and social costs or benefits leads to market failure and resource misallocation.

Common mistakes and how to put them right

• Confusing private costs with social costs.

  Put it right: Social costs include both private costs and external costs; they are only equal if there are no external costs.

• Assuming external benefits only apply to public goods.

  Put it right: Recognise that external benefits can arise from the consumption of merit goods, such as education and healthcare, which are private goods.

• Failing to identify the third party in an externality scenario.

  Put it right: Specify who the third party is, such as local residents or the healthcare system, when explaining external costs or benefits.

Exam tips

• Use clear equations, such as Social Cost = Private Cost + External Cost, to demonstrate your understanding of the relationships between these concepts.
• Always provide concrete examples of external costs, like air pollution, and external benefits, like vaccinations, when defining them.
• When analysing market failure, explicitly state whether the market is over-producing or under-producing the good relative to the socially optimum level.